I would guess it's something to do with the fact that most white British people who live in this country are just here because they were born here. On the whole there's no reason to expect them to be particularly good or particularly bad, academically.
On the other hand, people from some ethnic minorities are here because, they (or their ancestors) had performed so well academically in their home country, that they were accepted into top universities or top jobs here, and moved here in search of a more prosperous life. For someone living in a foreign country it takes quite a lot to be able to come here on that basis.
Of course you also get some ethnic minorities that perform below average academically in this country. Perhaps they (or their ancestors) came here as refugees from war-torn countries, or as cheap labour etc. in which case performing well academically might not be as culturally high up on their list of priorities.